1. Technical Field
The present invention relates to control of a linear motor.
2. Related Art
In recent years, in axial driving of a machine tool, a method which directly drives a table with an electrical motor (hereinafter, the electrical motor will be simply referred to as a “motor”) and without the use of a ball screw has been commercialized. In the machine tool of this type, a linear motor is used.
In the driving method using a linear motor, because there is no speed reducing element such as the ball screw, the positioning precision for a slider of the linear motor directly corresponds to a positioning precision of the driving axis. Therefore, a high positioning precision is demanded for the linear motor. Normally, when a linear motor is used, a high resolution position detector is mounted to detect a position of the driving target.
A linear motor comprises a slider which is mounted on a mobile unit such as a table and a stator which is mounted on a fixed unit such as a bed. There are various types of linear motor depending on the principle. For example, as described in JP 2007-318839 A, there is a type where a stator having salient poles arranged with a predetermined pitch on surfaces which oppose each other is used. On a stator which is provided extending in a straight line shape, salient poles are arranged along the direction of extension of the stator. On the slider which can move along the stator which is provided in an extended manner, magnetic poles are provided opposing the salient poles of the stator. The magnetic pole is formed by a coil and a permanent magnet. When a predetermined electric power is supplied to the coil, a moving magnetic field is formed in the direction of extension of the stator, and the slider moves in the direction of extension of the stator due to an interaction between the moving magnetic field and the salient pole.
Next, a typical method of controlling a linear synchronous motor will be described. FIG. 1 is a block diagram showing a structure of a control device of a linear motor. The control device of the linear motor comprises a position detector 12, proportional amplifiers 21 and 22, a current distributor 23, an integrating amplifier 24, a current controller 25, a differentiator 26, a 3-phase PWM inverter 28, and a current detector 29. In this control device, when a position instruction X is input, a difference between the position instruction value and a detection value of the position detector 12 which represents the position of the slider is amplified by the proportional amplifier 21, and is output as a velocity instruction V* of the slider of the linear motor. A difference between the velocity instruction V* and a velocity of the slider obtained by differentiating the detection value of the position detector 12 by the differentiator 26 is PI-calculated by the proportional amplifier 22 and the integrating amplifier 24, and a thrust instruction F* is generated. Upon receiving input of the thrust instruction F*, the current distributor 23 generates, of the 3-phase current instructions Iu*, Iv*, and Iw*, current instruction values Iu* and Iv*, and outputs these instruction values to the current controller 25. In this process, when the current instruction is generated, the detection value by the position detector 12 is taken into consideration.
The current controller 25 generates 3-phase voltage instructions eu*, ev*, and ew* based on the current instructions Iu* and Iv* which are input from the current distributor 23 and a current instruction Iw* derived from a relationship equation Iu*+Iv*+Iw*=0, and outputs the voltage instructions to the 3-phase PWM inverter 28. The linear motor is driven by applying 3-phase alternating current voltage converted by the inverter 28 from the direct current voltage, supplied from a direct current power supply 27, based on the 3-phase voltage instructions eu*, ev* and ew*. The voltages actually applied to the linear motor are the 3-phase voltage instructions eu*, ev*, and ew* determined by the current controller 25 from the differences from the current detection values iu, iv, and iw detected by the current detector 29.
In the above-described control method, in order to maximize the thrust of the linear motor with the same current, the current must be applied with an optimum current phase angle corresponding to the position of the slider, in particular, the position of the magnetic pole of the slider with respect to the salient pole of the stator.
A phase angle which is the optimum current phase angle will now be described. FIG. 2 is a diagram showing a thrust generated by the linear motor when the slider is slid with respect to the stator with a constant current. It can be understood from FIG. 2 that the thrust of the linear motor is maximized when the current phase angle is 90°.
In actual operation, the control device of the linear motor applies a current to the coil of the magnetic pole to achieve the current phase angle of 90° while monitoring the position of the slider obtained by the position detector, that is, the position of the magnetic pole of the slider. In this process, in order for the control device to apply the current at an accurate current phase angle, the position detector, the slider, and the stator must be mounted at predetermined positions. If the mounting position is deviated, the position detector cannot accurately detect the position of the magnetic pole of the slider, the current is deviated from the optimum phase angle, and the thrust is reduced.
In reality, however, the linear motor and the position detector cannot be accurately mounted at the predetermined positions because there exist a size tolerance of the components and backlash of the mounting bolt and the bolt hole, and thus, it is difficult to apply the current at the optimum phase angle.
More specific description will now be given. A current phase of 360° of a linear motor having the characteristic shown in FIG. 2 is assumed to correspond to a mechanical phase (pitch) of 12 mm. Under this condition, if the mounting positions of the slider, stator, and position detector are deviated by 30° (1 mm in mechanical phase) from the predetermined positions, as shown in FIG. 2, the thrust is reduced by 19% from the maximum thrust which can be obtained when the current is applied at the optimum phase angle. In order to prevent such a reduction of the thrust, as disclosed in JP 2008-178237 A, a magnetic pole position correcting method is proposed in which a magnetic pole position correction value is stored in the control device, and the position of the magnetic pole of the position detector is electrically corrected.
In large-size machine tools in which a driving stroke of the linear motor is very long, a stator which is divided into a plurality of portions may be used. That is, a stator may be formed by arranging a plurality of stator segments which are the divided portions, according to the length of the stroke. Due to the length size error and the mounting error of each stator segment, the pitch of the salient poles of the stator would be deviated between the front end and the rear end of the stator. In the magnetic pole position correction method of the related art, because there is only one magnetic pole position correction value, locations where the thrust is reduced are created depending on the position on the stator.
More specific description will now be given. It is assumed that, in the linear motor shown in FIG. 2, the current phase of 360° corresponds to a mechanical phase (pitch) of 12 mm, the stroke of the stator is 9000 mm, and 15 stator segments of the linear motor each having a size of 600 mm are arranged. Under these conditions, if the length size error of the stator is 0.1 mm and the mounting error is 0.1 mm, the pitch deviation of the salient pole of the stator at the stroke end with respect to the stroke center is at maximum 42° (or 1.4 mm in the mechanical phase). Therefore, when the overall region of the stroke is controlled using the magnetic pole position correction value determined at the center of the stroke, as shown in FIG. 2, the thrust obtained at the stroke end is reduced by 26% compared to the thrust obtained at the stroke center.
The stators divided into a plurality of portions include, in addition to the physically divided structure, a structure in which the stator is formed part by part in the manufacturing process. The stator is formed by layering steel sheets on one side of which recesses and projections are formed, and the projected portion becomes the salient pole. The steel sheet having the recess-and-projection shape is manufactured by stamping with a punch having a corresponding recess-and-projection shape. In the stamping, machining is executed a plurality of times while the punch is relatively fed with respect to a single material. In general, the punch is fixed and the material is sequentially fed into machine a plurality of times. After the stamping is executed once, the material is fed by the size of the punch, and the second stamping is executed at this position. These processes are repeated, to manufacture the steel sheet which is longer than the size of the punch and which has the recess-and-projection shape. In the following description, in addition to the portion of the stator which is physically divided, the portion of the stator in which the recess and projection are formed by a single stamping is also described as a “stator segment”.
The stator formed by executing stamping a plurality of times on a single material has a structure in which a plurality of the stator segments each corresponding to a single stamping are arranged in the direction of extension of the stator. In such a case where virtual stator segments are arranged also, the pitch of the salient poles of the stator may be deviated due to position deviation or the like between the stator segments. For example, the position of the salient pole may be deviated due to a size error of the punch in the movement direction of the material or an error in the amount of movement of the material for each machining.
An advantage of the present invention is that a control method having a reduced thrust reduction over the entire length of the stator is provided.